The present invention relates to processes for forming barrier layers on metal surfaces. It finds particular application in conjunction with forming barrier layers on titanium and aluminum containing substrates, particularly titanium aluminides, which resist oxidation, resist corrosion, resist wear and abrasion, and resist corrosive media.
Titanium aluminide is currently being investigated to replace super alloys for use in aircraft turbine engines and aircraft structures. Titanium aluminide is about half the density of superalloys of comparable strength, so a large reduction in aircraft weight is possible. The titanium aluminide alone is quite brittle, but workers have been able to add other elements to reduce this brittleness. A remaining development problem is that the oxidation resistance of these titanium aluminide compounds is lower than desired at elevated temperature. Therefore, a key factor in increasing the maximum use temperature is the enhancement of oxidation resistance while maintaining creep and strength performance.
Previous attempts to develop a protective coating have resulted in coatings which are unstable or tend to peel off. If a titanium aluminide substrate is oxidized in air or oxygen at high temperature, as is conventionally done, Al2O3 and TiO2 are formed. These two oxides have different structures and are immiscible in each other. As such, the mixed oxide is porous and weakly bonded to the substrate. Therefore, they are subject to spallation from the substrate. As such, the oxides are not an effective oxygen barrier. That is, they do not prevent the diffusion of oxygen into the substrate and the reaction of oxygen with aluminum, titanium, and other elements below the surface.
The present invention relates to a new and improved technique for forming strongly-bonded surface barriers for titanium aluminide substrates, which overcomes the above-referenced problem, and to the structures produced by such a technique.
In accordance with one aspect of the present invention, a process for the formation of a specific reactive element bilayer barrier on a titanium aluminide substrate is described. The bilayer barrier comprises an oxide coating formed from the reaction of specific reactive elements within the substrate with oxygen from water vapor in the presence of hydrogen at high temperatures and low oxygen concentrations. This coating is formed by placing at least a surface and contiguous regions of a titanium aluminide material in a gaseous atmosphere with a small concentration of water vapor at a high temperature. That temperature and water vapor concentration are then maintained throughout the oxide formation. The specific reactive elements at the substrate surface are reacted with oxygen from the gaseous hydrogen/water vapor atmosphere to form the bilayer barrier. The barrier layer is strongly bonded to the surface with an aluminum oxide at a substrate/barrier layer interface and a titanium oxide at a barrier layer/gaseous interface.
In accordance with another aspect of the present invention, the product of the process described above is provided. The product is a bilayer oxide coating on a titanium aluminide substrate. The bilayer oxide coating comprises an aluminum oxide layer on the substrate/barrier layer interface and a titanium oxide layer on the barrier layer/substrate interface.
One advantage of the present invention is that it provides a barrier that is resistant to permeation by oxygen.
Another advantage of the present invention is that it forms a barrier which resists wear.
Yet another advantage of the present invention is that it forms a surface barrier which inhibits erosion.
Still another advantage resides in the strong adhesion of a barrier layer to a titanium aluminide substrate.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.